fix: check near

[strasdat]

• implement FARM_NEAR according to best practices
  • in a symmetric form such that relativeCloseness(x, y) == relativeCloseness(y, x)
  • detecting NANs and PANIC in that case
• adding in FARM_ABS_NEAR if absolute comparison is needed instead: |x-y| <= thr
• fix newly detected bug (test was erroneously passing for a NAN value)

Fixes / clarifies issue discussed in this comment: #89 (comment)

[isherman]

I'm not crazy about the names in the public API: "FARM_ASSERT_NEAR" and "FARM_ASSERT_ABS_NEAR".

I think it's because I would use the "FARM_ASSERT_ABS_NEAR" whether or not I was dealing with numbers that were "near" each other. That's the use case that originally confused me about the API.

Here's another example:

double t_a = getTimestamp("a");
double t_b = getTimestamp("b");

// Sanity check that event a occurred within an hour of event b
FARM_ASSERT_ABS_NEAR(t0, t1, 60 * 60);

Could we follow the convention used in Catch2 and name them FARM_ASSERT_WITHIN_ABS and FARM_ASSERT_WITHIN_REL?

[strasdat]

@isherman thanks for the naming suggestion, I adopted them as proposed.